Hey, it's possible
Try this way Use the scroll bar on the right hand side to go to the music you've just added to the iTunes Library. Then drag the album or song onto your iPod listing under "Devices" in the left-hand column. If it's an album, the copy could take five minutes or more, so wait for it. After the transfer, the iTunes message box will say "iPod Sync is complete" even though you haven't used Sync to do the job. At the end, click on "Eject iPod", wait for the "do not disconnect" message to clear, go ahead and disconnect the cable. And you'll have all your old collection with the addition of your new music.